Field
Exemplary embodiments of the present disclosure relate to a novel method of sonoelectrolysis for metal removal.
Discussion
Removal of heavy metals from industrial wastewater is of primary importance because they not only cause contamination of water bodies but are also toxic to flora and fauna. Industrial processes generate wastewater containing heavy metal contaminants, which invariably are toxic and non-degradable. Heavy metals cause damage to the nervous system and the kidney. Heavy metals toxicity also causes cancer and other metabolic disturbances. The metals of most immediate concern are chromium, lead, zinc, iron, mercury and lead. The concentration of these metals must be reduced to acceptable levels before discharging them into the environment.
The typical heavy metals discharging industries are electroplating industry, tanneries, battery industry, and metallurgy, etc. Electroplating wastewater is by far the most important environmental problem in developing countries as it is highly polluting utilizing a variety of chemicals. Similarly, tanneries discharge highly polluted wastewater in terms of chemical oxygen demand, total suspended solids, chromium, copper, iron and zinc, well above the maximum standards established worldwide putting extensive strain on the environmental control efforts in the developing world.
A major challenge facing humanity today is to provide clean water to the population around the world, particularly in the developing countries, where fast development of industrial infrastructure has heavily taxed the supply and quality of water. There is, therefore, an urgent need to develop innovative, more effective and affordable techniques for waste-water treatment that will allow recycling of water and reduce the health hazards from contamination of industrial effluents.
A wide range of wastewater treatment techniques are currently used including biological processes for nitrification, denitrification, phosphorous removal, as well as a range of physicochemical processes requiring chemical treatment. The commonly employed physicochemical treatment processes used in the water industry are micro and ultra-filtration, ion-exchange (anionic and cationic), chemical precipitation and oxidation, carbon adsorption, reverse osmosis, electro-dialysis, and air and gas stripping, and volatilization. Many of these techniques are expensive to use.
Novel techniques that expedite removal of contaminants from water supply are widely studied; for example, sonoelectrochemistry has also been proposed for the treatment of toxic wastes since it offers several advantages. It has been suggested that the removal of phenol from industrial effluents by electrochemical oxidation is accelerated in the presence of ultrasound. It is possible to induce almost 80% oxidation of phenol to maleic acid when ultrasound is applied compared to less than 50% when ultrasound energy is not used. Hydroxyl radicals appear to be the main active reagent that reacts with the organic compound, whose oxidation can be enhanced by combining various traditional techniques (e.g., O3/H2O2, UV/H2O2, ultrasound/O3 and UV/H2O2/ultrasound). Sonoelectrochemistry can also be employed in the disinfection of sewage and potable water. For example, in the water industry, chlorine disinfection has proved to be successful in eradicating water-borne diseases (e.g., those caused by cryptosporidium and E. coli). Chlorine is often produced on-site by electrolyzing hydrochloric acid and thus helps in the disinfection of environmentally toxic effluents. It has been found that electrolyzing 22% hydrochloric acid, approximately 59% of chlorine was evolved in the presence of ultrasound compared with 1% in the absence of ultrasound. Thus sonoelectrochemical waste treatment may reduce energy requirement in removing environmental pollutants from water.
Other promising techniques based on electrochemical technology have been developed but are not yet been commercialized. One of these processes is known as electrocoagulation, an electrochemical method of treating polluted water and effluents whereby sacrificial anodes oxidize (or corrode) to release active coagulant precursors (usually aluminum and/or iron ions) into the solution. In other words, the coagulant is generated ‘in-situ’ by electrolysis. Electrocoagulation has a long history as a wastewater treatment technology having been used for the removal of a wide range of pollutants (mainly inorganic and organic components). However, electrocoagulation has never become accepted as a ‘mainstream’ water treatment technology due to the difficulties in designing a practical electrocoagulation reactor mainly due to the issue of electrode reliability (particularly passivation of the anode over time also called ‘electrode fouling’).
Whereas the removal and recovery of heavy metals is normally accomplished by adsorption, ion exchange, chemical precipitation, membrane separation and electrochemical deposition, the efficiency and the cost of removing low level contamination remains high and often out of the reach of countries where the problems are more severe.